Signal voltage limiter



Nov. v12, 1935. w. L. 'DOUDEN SIGNAL VOLTAGE LIMITEE Filed April 1'7, 1950 lNvENoR wLLlAM L. DOUDEN ATTRNEY Patented Nov. 12, 1935 2,020,825

UNITED STATES PATENT OFFICE SIGNAL VOLTAGE LIMITER William L. Douden, Brooklyn, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application April 17, 1930, Serial No. 444,968

I 7 Claims. (Cl. Z50-20) My present invention relates to signalling arpulses above a predetermined minimum intensity rangements, and more particularly to signal voltvalue. age limiting devices adapted for use in such ar- Another object of the invention is to provide, in rangements. a receiving circuit, a detector provided with It is extremely desirable to utilize devices, in means for limiting its operation to electrical im- 5 a signalling system, the primary function of pulses above a predetermined minimum inten- Which is to limit the sensitivity of the system to sity value whereby background noises and faint predetermined maximum and/or minimum: sgsignals below this value are rendered substannal voltage values. For example, in operating a tially inaudible, vthe latter result being particu- 10` receiving circuit in the design of which sensitivity larly advantageous while tuning the circuit 10 has been stressed, it is quite apparent that the through portions of the broadcasting range conreception of signals from strong local broadcasttaining signals of intensities which are too faint ing stations, and the existence of a background for the said circuit. noise level, presents a disturbing, and often an- Still other objects of the invention are to imnoying, problem to the receiver operator.v Again, prove the efficiency of receiving systems in genl5 a similar situation exists in receivers of relatively eral, and to provide a circuit in particular, which greater selectivity and somewhat lesser sensiis not only reliable in operation, and free of untivity. To illustrate, the highly sensitive superdesirable characteristics, but economically proheterodyne receiver possesses sumcient sensitivity vided with the devised improvement.

to amplify background noises, and faint unde- The novel features which I believe to be charzo sired signals from distant points, to an annoyacteristic of my invention are set forth in paring extent. Obviously, the strong signal from a ticularity in the appended claims, the invention local broadcasting station presents the same obitself, however, as to both its organization and jection, in this case, as in the less selective remethod of operation will best be understood by ceiver. reference to the following description taken in 25 Automatic gain control has been resorted to connection with the drawing in which I have in an effort to solve the problem of the strong indicated diagrammatically one circuit arrangelocal signal. But, it is readily realized that this ment whereby my invention may be carried into type of control may be inadequate, for it deeffect.

pends on regulation by .carrier frequency inten- Referring to the figure in the drawing, a super- 3o sity to eliminate background noises, since the heterodyne receiving circuit is diagrammatically latter are greatly intensified when a faint carshown embodying one form of the invention, the rier frequency, or none at all, is being detected. circuit including means A, G for collecting signal Now, I have devised a method of, and means energy, and other electrical impulses. The col- ;5, for, controlling the operation of a signalling lecting means is coupled, as at M, to a tuned system, such as a receiving circuit, the method radio frequency amplier, the latter being conof control functioning to provide two limits beventionally shown since any type, well known to yond which the system does not respond thereby those skilled in the art, may be utilized. determining the maximum intensity of signal to The amplified radio frequency energy is trans- .0 be detected, and, additionally, definitely estabmitted to a frequency changer, through a coullshing aminimum intensity of signal which will pling M', the radio frequency energy being heteroperate the detecter. odyne in the frequency changer with energy from Accordingly, it is one of the main objects of a local oscillator, supplied through the coupling my invention to provide a method of, and means Mo. As is well known to those skilled in the art,

5 for, limiting to predetermined maximum and/or the beat frequency is superaudible, and is desig- 45 minimum intensity values, the signal voltage to nated as the intermediate frequency. The latter be detected by a high frequency receiving circuit. is transmitted, through a coupling M", to the Another important object of the invention is intermediate. frequency amplifier, the amplifier to provide in a superheterodyne receiver includbeing xedly tuned by the coil L and capacity 3 ing an amplifier of super-.audible beat frequen- C to the intermediate frequency. 5o cies and a detector, means associated with the While I have shown the intermediate amplifier amplifier to limitfthe signal fed to the detector as including only a single electron discharge deto a predetermined maximum intensity, and advice l, it is tobe understood that a plurality of ditional means associated with the detector to such devices may be employed to increase ami limit the operation of the latter to electrical implication. The anode of the device I receives potential from a source B, through the primary coil of the coupling transformer T. The amplied intermediate frequency is detected by a subsequent detector stage, the latter including the electron discharge device 2.

Potential is supplied. to thecanode of ldevice 2, from a source B', control electrode detection being employed by connecting Aa capacity 5 in series with the control electrode, andshunting a leak resistor E 'across the capacity. The output of the device 2 is transmitted to an audiofreas a regulating, limiting or controlling means,

Vfed back along path, i is alinear feeds energy from the anode circuit to the control electrode circuit, in phase opposition tothe Y input energy, thereby constantly exercising a limiting action on the amplified output voltage.

The relation between output and Ainput v-oltage of device I is onlylinear for a portion of the input voltage increase, and, thereafter, for increases of input voltage no further increase of output occurs. On the other hand, as the relation between the input voltage and the voltage one for all values of input voltage, it will be readily appreciatedlthat there exists a certain value of input Y voltage beyond which further increase ci input voltage results in a decrease of output voltage. This necessarily results because of the fact, as statedheretofore, that the voltage in the anode circuit of the device I is in phase opposition to the voltage `in the control electrode circuit of the V-deviceL Y atVY 't It is, therefore, seen that the resultant output voltage is secured by subtracting, for all values of input voltage, the output energy-'fed back along path`3, ll from the normal value of output voltage atv any'given input voltage value. By making the resistor 3 adjustable, the value of the limiting maximum intensity can beevaried. In this waiathe operation of theintermediate frequency amplieris controlled so that the elec- `trical energy fed to the detector cannot exceed a predetermined maximum intensity value. Y Obviously, one important advantage of a limiting arrangement is in connection with local reception,itfbeing possible to limit the intensity of local signalsfto a desired maximum value, thereby eliminating blasting and similar objections.

To'limit 4the detecting action of the device 2 and itsassociated circuits, asource of biasing potential 'l is connected to the control electrode of the device 2 so as to constantly maintain a predetermined, negative biasing potential on the said contro-l electrode. This is -accomplished by connecting the negative terminal of the' source 'I toY one Vterminal of aY potentiometer resistor 8, and connectingthe positive terminal lof the biasing source to the other terminal of the resistor, the positive Y'terminal being, also, connected to the lead ofthe cathode of-'device 2. -A `sliding Contact member 9 is arranged for sliding cont-act with the resistor 8, and is, additionally, connected to one terminal of the Vleak resistor 6.

f Thus. it will'be seen that the control electrode control electrode of can be normally biased to any desired negative potential by adjusting the sliding contact 9. In this way, a limiting minimum intensityV value is readily established for the detector operation.

As a consequence, electrical impulses, Vwhether 5' faint signals or background noises, having an intensity below this aforementioned minimum limiting value, will not be detected, and subsequently be rendered inaudible to ther listener. For signals just audible, a slight increase in the 10 grid bias on the control electrode oi device 2 renders the signals-inaudible, and if the grid bias Y is increased 'suflciently everything, except the high power local stations, disappears completely. Y

There is a certain'value of bias at which the background noise is substantially inaudible-but even an extremelyfaint signal comes through, ratherclearly; Vbelow this value of grid bias an Y appreciable increase in background noise occurs without any appreciable increase in the applrent sensitivityV of the set to broadcast signals. It may bepointedout that the value of the leakV resistor 6 is Ychosen so that at the critical'point,V above referred to where the backgroundV noise is just eliminated, the limiting arrangement operates efficiently toperform the latter'functior'i.V

It will, 'thu-s, be seen that the arrangement disclosed herein, makes it possible-to definitely establish, in a vpredetermined manner, limits beyond which the detector will not be called upon to operate, and below which the electrical impulsescollected will not -operatethe detector, the signal-intensity between'these limits being regulated in any desired manner. For example, the usual Aautomatic Volume control can be employed between these limits so as to maintain a uniform intensity level. It is readily noted that the operation of an automatic volume controlarrange--` Y ment, inconnection with a receiving circuit, is greatly enhanced withethe addition of theimprovements disclosed herein. i e Again,'it is pointed out` that the invention is notflimited to alrsuperheterodyne receivingcir-` cuit, but the limiting larrangements disclosed may be employed on any well known type of nonheterodyne Vtuned radio frequency receiver. thermore, ,electron discharge devices, other than triodes, maybe employed'in placecf thedevices I, 2, such devices being, for example, screen grid tubes. Again, it is to be understood that the path 3, 4 can be connected between the control electrode ofthejdevice I and the `high potential end of the secondary coil of coupling means T, it Y being necessary in such a. case toinsure feedback of energy*Y in phase opposition to the VVinput`l energy ofthe device Vel, the latter being accomplished inA any manner well knownl to those skilled in themart. "Limitation of outputfofthev amplifier Vmay also bepr'oduced' by limiting [the electron emission o-f thecathodefof the amplifier, While I have indicatedl and described one arrangement for carrying my invention into effect, itwill be apparent to one skilled inthe art that `my, invention is by no` means limited to the particularjorganization shown and described,A but 65,r

and electrically independent of said iirst means to limit the operation of the detector to electrical signal impulses above a predetermined minimum value whereby faint signals and background noises below the said value are not detected.

2. A high frequency receiving circuit comprising anamplier and a detector having means associated with the amplier to limit the signal fed to the detector below a predetermined maximum intensity to prevent blasting, and means associated with the detector and electrically independent of said first means to limit the operation of the detector to electrical signal impulses above a predetermined minimum value, said second means consisting of a self-biasing means associated with the detector grid, and means for applying a predetermined biasing potential to said grid.

3. In a superheterodyne receiver, an intermediate frequency amplier, a second detector tube provided with an input and output circuit, means for establishing a limiting minimum signal intensity value for the detector operation comprising a source of biasing potential connected to the grid of the tube, an impedance in shunt with said source, the positive terminal of said source being connected to the tube cathode, a capacity in series in the grid circuit, and a grid leak resistor connected in shunt with the capacity and between said grid and a desired point on said impedance.

4. A method of receiving broadcast signals with a tunable radio receiver having an amplifier and detector which consists in collecting broadcast signals of a desired frequency, amplifying said collected signals, automatically regulating the amplification to prevent the input to the detector from increasing above a predetermined maximum signal intensity Value, detecting the amplified signals, and rendering the detection inefiicient without effect on said amplification regula-tion when the receiver is tuned through a portion of the broadcast range where signal energy below a predetermined intensity value is collected.

5. In combination with a radio receiver provid- 5 ed with automatic volume control means for maintaining the signal input to the detector less than a predetermined intensity value, means electrically independent of said control means for normally biasing the grid of the detector negative with respect to the cathode, said negative bias, being of a magnitude such that the detector is rendered inoperative to detect undesired background noises when the desired signal energy decreases below a predetermined intensity level without affecting the operation of said control means, and additional means for adjusting the magnitude of said bias whereby the background noise level may be controlled.

6. In combination with a radio receiver which is provided with a signal amplier having tuning means, a signal detector adapted to feed a reproducer, and automatic volume control means for maintaining the signal amplitude at the detector input less than a predetermined amplifier signal input amplitude, means for applying a normal bias to the detector of such magnitude that the detector is rendered inoperative to detect undesired background noises when the signal amplitude decreases below a predetermined value, whereby said noises are not reproduced when the tuning means is adjusted through a portion of the tuning range where signals below said amplitude value are collected, said applying means being electrically independent of the volume control means whereby the operation of the latter is unaiected by changes in the detector operation.

7. In a radio receiver of the type defined in claim 6, said bias applying means being adjustable to vary the magnitude of the said bias.

WILLIAM L. DOUDEN. 

